This invention relates to an electromagnetic interference ("EMI") shielding device for a video display unit ("VDU") and methods for making the same. More particularly, this invention relates to an EMI shielding device in which an electrically conductive transparent screen or a transparent conductive coating is applied to the face plate of the cathode ray tube (CRT).
It is well ,known that operation of a cathode ray tube results in a build up of static electricity on the face or display portion of the cathode ray tube. This build up of static electricity causes a disadvantageous accumulation of foreign particles on the face plate of the video display unit as well as the VDU operator. In addition, it is advantageous to provide an antiglare treatment for the viewing surface of VDU displays. Antiglare treatments are typically accomplished by causing the outermost viewing surface to have a slight texture (non flat) surface. In many applications, this antiglare feature is achieved by mechanically or chemically etching a textured surface into the viewing face of the cathode ray tube itself. This process may be relatively expensive. In other antiglare applications, a texture is imparted to a face plate of rolled plastic which may be sold separate from the VDU as a feature added by the user.
It has recently become known that EMI shielding may be provided by knitted or woven wire mesh which is fully laminated between glass and/or plastic substrates. For example, see the brochure by Chomerics entitled "Chomerics Optical Products", date unknown. While this may be effective means for reducing EMI and the build up of static electricity on the face of a video display unit, shielding of this type is typically for military/aerospace application and it requires relatively complex manufacturing procedures.
The manner in which an EMI shielded face plate is mounted to the bezel of the CRT housing is important for many reasons, including grounding efficiency, mechanical stability, and simplicity of construction. In particular, it is known that the edge of a shielded face plate may be surrounded by a conductive busbar which contacts the conductive coating or the screen mesh of the face plate to make electrical contact to the enclosure bezel. The window is then typically bonded directly to the bezel using a conductive room temperature vulcanization adhesive or a conductive epoxy. When an adhesives such as these are used, several mounting clips are typically applied to the face plate to assure bonding. The manner in which the shielding is grounded to the bezel is also important in the production of effective shielding windows. One prior art method for grounding the face plate is to permanently bond the shielding screen to the enclosure by spot welding, brazing, soldering or elastomeric compression bond. In other applications, a conductive metallic or elastomeric gasket is mounted to the face plate in contact with the wire mesh screen or conductive coating. The primary function of this gasket is to minimize the EMI noise which is produced when the EMI currents in the mesh or conductive coating are returned to the system ground.